[0037] Rename NNNN-assets to 0037-assets (microsoft#540)

This was an oopsie.

Author: llvm-beanz

proposals/NNNN-assets/cbuffer-grammar.png proposals/0037-assets/cbuffer-grammar.pngproposals/NNNN-assets/cbuffer-grammar.png renamed to proposals/0037-assets/cbuffer-grammar.png

@@ -83,7 +83,7 @@ grammar formation is adopted for cbuffers:
   empty-declaration\br
 \end{grammar}
 ```
-![Latex Rendering](NNNN-assets/cbuffer-grammar.png)
+![Latex Rendering](0037-assets/cbuffer-grammar.png)
 
 This simplified grammar disallows members of `cbuffer` declarations that do not
 have semantic meaning, and allows a simplification of `cbuffer` scoping rules.